(a) Field of the Invention
The present invention relates to a gap eliminator used for reducing working cycle time in machine tools.
(B) Field of the Prior Art
Conventional machine tools have sometimes incorporated therein a circuit which serves as a gap eliminator to eliminate air cut time. In such circuit, only variations in electric power have been taken into consideration, while the scattering of power during idle run (i.e., run when no grinding is being performed) is left unconsidered. Therefore, a reference circuit with power during idle run taken as a reference must be incorporated before such eliminator circuits can be applied to all types of machine tools. In a gap eliminator for reducing working cycle time, when a system for transmitting signals using power is used, periodic noise is produced owing to a belt or the like used, so that the setting of a reference level is difficult. The level of this periodic noise is about 5-15 Hz and elimination of this noise becomes necessary. Now, while it is possible to eliminate this noise only by a low-pass filter, it is known that the response of low-pass filters delays by an amount of about t.sub.1 (=CR) seconds, where C is a capacitance and R is a resistance. For example, when it is desired to reduce a periodic noise of 5 Hz to one-tenth, then t.sub.1 = 0.32 seconds. When a gap eliminator is used, this delay of t.sub.1 seconds will result in a workpiece being worked in quick feed for t.sub.1 seconds, thereby destroying the inherent function of the gap eliminator.
As concrete examples of known gap eliminators, there are circuit arrangements shown in FIGS. 1 and 2 for machine tools utilizing electric power, wherein in a cutting, grinding or other machining operation, approach feed effected between the tool rest and the work table is made variable and means is provided for eliminating air cut time which does not contribute to substantial machining operation. The apparatus shown in FIG. 1 is arranged as follows. An unadjusted output .function.(t) + a sin .omega.t from a power detecting circuit arranged to detect the electric power consumed by an electric motor for driving a grinding wheel is inputted into a switching circuit. When a signal from a limit switch (not shown) for ascertaining a return end for the grinding wheel rest is given, the output .function.(t) + a sin .omega.t from the power detecting circuit is inputted into a memory circuit through the switching circuit. As a result, the memory circuit stores a power consumption value .function.(t.sub.0) + a sin .omega.t.sub.0 indicated immediately before the grinding wheel leaves the return end, said value being then fed into a comparator circuit. In this comparator circuit, a power consumption value .function.(t) + sin .omega.t indicated when the grinding wheel is advancing has been inputted. Thus, the output deviation between the two input values, {.function.(t) + a sin .omega.t} - {.function.(t.sub.0) + sin .omega.t.sub.0 }, is fed into a Schmitt circuit and sent as an output to a feed control circuit. The feed control circuit has successively inputted thereinto a start command, an operation signal for the Schmitt circuit, a precision grinding signal and a fixed sizing signal and it is adapted to successively send a quick feed command, a rough grinding command, a precision grinding command and a stop signal to a feed device (not shown); it sends a quick return command a predetermined period after the stop signal has been sent.
The apparatus shown in FIG. 2 is arranged as follows. An unadjusted output .function.(t) + a sin .omega.t from a power detecting circuit is divided into parts under no load and loaded conditions, respectively, which are then fed to a comparator circuit. The output .function.(t) + a sin .omega.t under no load condition is fed to a memory circuit and a power consumption value .function.(t) + a sin .omega.t.sub.0 indicated immediately before the grinding wheel leaves a return end is stored. A predetermined value b from a setting circuit is added in an adder circuit, whose output .function.(t) + a sin .omega.t.sub.0 + b is inputted into the comparator circuit. On the other hand, a power consumption value .function.(t) + a sin .omega.t indicated when the grinding wheel is advancing is inputted into the comparator circuit by a switching circuit. Thus, the comparator circuit sends an output deviation between these two inputs, {.function.(t) + a sin .omega.t} - {.function.(t) + a sin .omega.t.sub.0 + b}, to a speed change control circuit. The manner in which a feed control circuit is controlled by said output is the same as in the apparatus shown in FIG. 1 and hence a description thereof is omitted.
Now, the two examples described above leave the following conditions unconsidered. First, power waves themselves have subtle chattering due to noise, and the presence of this chattering greatly influences the working conditions of gap eliminators, as described above. Second, owing to the elasticity of the grinding wheel and workpiece, there is sometimes a very small time lag in the value at the instant that the grinding wheel strikes the workpiece. Third, overall variations in electric power are not taken into consideration. Because of these problems, the present situation is that 100% satisfactory and extremely accurate control cannot be expected with respect to the control of the change of the feed rate of the grinding wheel.